The invention rose primarily out of the needs and concerns associated with mining the ore deposits of the Coeur d'Alene area in the North Idaho panhandle. The predominant economic base metal ores of this area are galena (PbS) and sphalerite (ZnS). One uneconomic ore also found in great abundance and in intimate association with these base metal ores is pyrite (FeS.sub.2). This mineral plagues the efforts of recovery and processing of rare earth metals, such as gold and silver, as well as the base metals of zinc and lead.
The presence of pyrite not only hampers recovery and processing procedures for metals, but it also presents the mining industry with a challenge to meet and maintain discharge water quality standards. A highly acidic mine water results from bringing together the necessary elements of oxygen, water and sulfide bearing minerals such as pyrite, sphalerite and galena. The mine environment is quite conducive to the generation of acid mine water through the oxidation of sulfur and iron to sulfuric acid and ferric residues, which is in part due to a bacteria catalyzed reaction of iron with water.
Hand-in-hand with the efforts to maintain water quality standards is the economic desire to recover the values in these ores. A potential economic source of zinc can be found as soluble zinc and iron flowing in the acid mine drainage of both active and inactive mine environments. The source of this soluble zinc and iron is from water resulting from the condition created from a naturally occurring in situ bacterial leach from overburdened deposits of the pyrite and sphalerite. Water percolating through the broken refractory ore strata initiates a biochemical oxidative reaction between a predominantly endemic bacteria, thiobacillus ferrooxidan, and the ore.
Pyrite oxidation is a model system for discussing the chemical and biochemical reactions responsible for the creation of acidic waste water flows. The following expressions are generally considered as accurate representations of the overall chemical/biochemical behavior of ore body leaching. EQU 4FeS.sub.2 +15O.sub.2 +2H.sub.2 O--(bacterial catalyzed).fwdarw.2Fe.sub.2 (SO.sub.4).sub.3 +2H.sub.2 SO.sub.4 EQU 4FeSO.sub.4 +O.sub.2 +2H.sub.2 SO.sub.4 --(bacterial catalyzed).fwdarw.2Fe.sub.2 (SO.sub.4).sub.3 +2H.sub.2 O
These reactions can and do occur as purely chemical oxidations. However, the bacteria are autolithotropic and utilize the iron as an electron source to drive their own metabolic machinery and in so doing act as catalysts by lowering the activation energy of the reaction. This lower activation energy in turn accelerates the reactions to generate product.
The subsequent reaction in the interior of the mineral is anaerobic in nature and occurs as follows: EQU 7Fe.sub.2 (SO.sub.4).sub.3 +FeS.sub.2 +H.sub.2 O.fwdarw.15FeSO.sub.4 +8H.sub.2 SO.sub.4
As can be observed from the above, a by-product of sulphate (SO.sub.4) is liberated and contributes to the increased acidity of the percolating water.
With acid pH conditions under 2.5, both zinc and iron are relatively soluble and therefore are in solution. At a pH of approximately 2.67 or above, Fe.sup.3+ at 10.sup.-4 molar concentration (a typical concentration) becomes insoluble and precipitate as ferric hydroxide (Fe(OH).sub.3), which is commonly known as yellow boy. The higher the iron concentration, the lower the pH at which ferric hydroxide will precipitate. Throughout this document, reference will be made to "ferric hydroxide" while it is to be understood that the true nature of this precipitated material could be in the form of Fe.sub.2 O.sub.3.nH.sub.2 O.
Ferric hydroxide is a stubborn gelatinous material that is difficult to remove from acid mine drain water. Presently, the most cost effective prior art methods for removal of precipitated ferric hydroxide is by flocculation with polyionic polyacrylamides. Slurried calcium oxide (CaO) is mixed with the mine discharge water and tailings from the metals processing mills to increase the pH to a range of 6.0 to 8.5. In this pH range, most processed by-product metals precipitate as hydroxides or carbonates. The flocculating agent provides a mechanism for entrapping the precipitated material through polyionic bonds and consequently increases its density beyond that of water. These compounds are pumped to settling ponds or sedimentation circuits to remove heavy metals and unwanted minerals. The water is then returned to the milling circuit or released to the environment. This prior art process takes weeks for proper settling and sedimentation, and requires significant capital a large area of land. Further, inherent in this prior art practice is the discarding of possibly significant amounts of potentially recoverable zinc.
Accordingly, a need remains for an improved process for removing yellow boy from acid mine water drainage.